This thesis is concerned with the problem of defining the processes by which a subject selects a response to a stimulus. Initially certain topics in two areas of research, sensory scaling and threshold measurement, are revived. In relation to the first the conflict between Fechner's law, which is derived from threshold measurements, and the more recent 'power law' given by 'direct methods' of scaling, is examined. It is shown that this conflict is due not to differences in the data from which the laws are derived, but to differences in the assumptions underlying the treatment of the data, and it is suggested that the logarithmic function may prove more useful for explaining a number of the findings in experiments on scaling, and that the same central effect of the stimulus say be considered to determine the response whether scales are being derived or thresholds measured. A short account of the development of models of the sensory discrimination process is then given, and evidence is reviewed which appears sufficient to justify applying the theory of signal-detection to the sensory threshold, and rejecting the neural quantum theory. To investigate the threshold mechanism further it was decided to examine the effect of an accessory stimulus given in controlled temporal relation to a critical stimulus on the threshold for the latter. The first experiment was an attempt to confirm Motokawa's finding (Gebhard, 1953) that sensitivity to an electric phosphene is affected a preceding flash of light, reaching a maximum 1-3 seconds after the flash, the exact time depending on the colour of the light. The absolute threshold for the electric phosphene was measured by the method of limits at intervals of 1-9 seconds after a flash of blue or white light. Motokawa found that sensitivity was maximal when the interval between the stimuli was 2 seconds for the white light, or 3 seconds for blue light. Neither particular could be confirmed. There was no peak of sensitivity in the range studied, nor was there any marked difference in the curves relating threshold level to inter-stimulus interval for the two colours of light. Instead a monotonic relation between threshold and inter-stimulus interval was found, for this range, the threshold falling as the inter-stimulus interval decreased, slowly at long intervals and more rapidly at the shorter ones. This raised two questions: why could Motokawa's findings not be confirmed? and what was the nature of the effect which had been found? The experimental technique of Motokawa and his colleagues is reviewed, and it is suggested that their findings mat be an artefact of certain features of their procedure, in particular the use of large steps where a threshold change is not expected, and small steps where a threshold is expected to appear. Two possibilities are considered for the threshold-lowering effect: it might depend on the use of two visual stimuli and reflect a peripheral interaction at the retina, or it might be a central effect, not specifically visual. To investigate this the light flash was replaced, in the second experiment, by the ring of a bell of the same duration and given at the same inter-stimulus intervals. The same threshold-lowering effect of the accessory stimulus was found as in the first experiment, showing that the effect could be produced by an accessory stimulus in another modality. Further support for this conclusion was provided by Experiment 3, in which both the light-flash and the bell were used, the former at the same intervals as in Experiment 1, but the latter always preceding the critical stimulus by 1 second. Here the threshold did not vary significantly: there was little or no residual effect of the light when the bell was given at a shorter inter-stimulus interval. It appeared that it was the interval of time since the most recent accessory stimulus, whatever its modality, that mainly determined the degree to which the threshold fell. In Experiment 4 the generality of the threshold-lowering effect was examined further. Using a visual accessory and auditory critical stimulus the possibility that the effect would occur only with visual critical stimuli was excluded. These findings raised two questions: What processes underlie the change in threshold? What determines the relation between threshold level and inter-stimulus interval? Possible answers to the first question were suggested in terms of the analysis offered by signal detection theory. In this theory it is assumed that the decision whether or not a stimulus has been administered is made by a process equivalent to comparing the afferent input with a criterion, and the criterion is computed in a way which may take account of the parameters, the variance (andsigma;<sup>2</sup>) and mean (M), of the 'noise' and 'signal + noise' distributions. The possibilities considered were that a change was produced in the functioning of the afferent paths at the time that the message from the critical stimulus was travelling along them, equivalent to a reduction in andsigma;<sub>N</sub> or M<sub>N</sub>, with a consequent change in the value of the criterion, c, computed by the subject (a 'distribution effect'), or that the computation of c was affected directly (a 'criterion effect'). Two hypotheses were proposed in answer to the second question (the relation between threshold level and inter-stimulus interval): (a) The accessory stimulus might have an 'arousing' or 'alerting' effect, causing an immediate central change with a small latency, which then decayed with time. (b) The subject might make use of the accessory stimulus as a 'temporal reference point' or 'warning'. By virtue of temporal information which he might possess he could use the accessory stimulus to determine when he could expect the critical stimulus to arrive. Here two subsidiary hypotheses might be suggested: (i) The subject lovers his threshold for the whole of the 'waiting time'. (ii) He may lover his threshold only when he expect the critical stimulus. Since his time-keeping ability is of limited precision, and the range of error will be greater for long times than for short, the 'range of expectation', if he allows for this, will be smaller the shorter the inter-stimulus interval. In either case, if the reduction in threshold is inversely related to the period for which it is reduced, a relation between threshold level and inter-stimulus interval of the sort found would be produced. In Experiment 5 an attempt was made to test the 'warning' hypothesis directly by using a small range of randomly varied inter-stimulus intervals instead of a single fixed interval as previously, in order to see whether a fall in threshold daring the 'range of expectation' could be demonstrated. Ranges of 0.5-1.5 seconds were used but no consistent effect was found. In Experiment 6 the same procedure was used but the inter-stimulus intervals were varied at random over a large range - 5 seconds - to prevent the accessory stimulus providing temporal information. Though it was found that the fall in threshold previously shown o^er this range disappeared, the possibility that this was due to 'habituation' of an 'arousal response' could not be excluded. The exact relation between the fall in threshold and the inter-stimulus interval was next considered.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:644607 |
| Date | January 1962 |
| Creators | Treisman, Michel |
| Publisher | University of Oxford |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
| Source | http://ora.ox.ac.uk/objects/uuid:2375133b-ee0c-48fe-ad22-2f9815d58ff2 |
Page generated in 0.0031 seconds